The LHC is a high energy high intensity proton collider presently under construction at CERN'

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Radiation Tolerant Commercial-Off-The-Shelf
components for the remote readout of RADFETs and
PIN diodes C. Pignard, Dr. T. Wijnands CERN
European Laboratory for Particle Physics, Geneva
23 1211 Switzerland
Introduction
Silicon Dosimeter Operation and Features
The LHC is a high energy high intensity proton
collider presently under construction at CERN.
A remote readout system for online radiation
measurement during beam exploitation will be
installed over the 27km long underground
accelerator tunnel. 244 silicon dosimeters
will measure radiation level IEL ( RADFETs )
NIEL ( PIN diodes ) and SEU counting (
SRAM sensors ). Dosimeters are accessed by 15
WorldFIP fieldbus segments Each segment could
drive up to 30 dosimeters. The dosimeter design
uses COTS components and can operate up to a
200Gy total dose.
Operation During irradiation, the RADFET is
short circuited (switch S2 closed) and the drain
is connected to the source. To readout the NMRC
RADFET (switch S1 closed), switch S2 is opened
and a constant current of 8.7mA crosses the
device during 5ms. The remote readout of 3
neutron PIN diodes such as the BPW34-FS (Siemens)
follows the same procedure excepted that the
readout current for these diodes is 1 mA. The 12
bits ADC is connected to the chipset interface
bus of a deterministic WorldFIP fieldbus. The
threshold and forward voltage are thermally
compensated during the readout.
HADRON Sensor Logic
WorldFIP Interface
 
S2
S2
S1
S2
Measurement features
Supervised Areas
RADFET PIN diodes Remote Readout
244 monitors at LHC startup. 255 junction boxes
distributed by 15 1MHzWorldFIP Fieldbus
segments. Total segment length 19Km
Lower side
Upper side
Sensor Window
Hadron Sensors ( SEU )
WorldFIP Interface
Neutron Sensors PIN diodes
Temperature Sensor
IEL Sensors RADFETs
Power Board
110V/220V Main Supply
Assembled
Main Board
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Radiation Tolerant Design and Measurements
Thermo compensated current source
Radiation Tolerance of Components
LM234-Z3 Zero Tcoef.(Temperature coefficient)
Current source The Iset current is the sum of
I1 I2, (each contributing approximately 50)
IBIAS. IBIAS is usually included in the I1 term
by increasing the VR value used for calculations
by 5.9. I1 VR/R1 and I2 (VR
VD)/R2 227mV/C as the Tcoef of the LM234
including IBIAS -2.5mV/C as the T.coef of the
diode BAS16. ISET I1 I2 dISET/dT dI1/dT
dI2/dT (227mV/C) /R1
(227mV/C 2.5mV/C) /R2 0 (
solve for Tcoef 0 ) R2 / R1 ( 2.5mV/C
227mV/C ) / 227mV/C 10.0
Single Event Latch-up Protection
The 12 bit A/D converter ADS774KU from Texas
Instruments was selected because this device
showed no sensitivity to latch-ups during an
irradiation test in the LHC radiation test
facility at CERN (TCC2). With an external 2.5
Volt voltage reference (AD680JR, Analog Devices),
this device provides reliable ADC conversion at
100 Hz up to 160 Gy TID. At 180 Gy the least
significant bit could not be switched on
anymore and the S/N ratio has decreased by
approximately 10. At this point, the current
consumption has increased by 30 ( green curve
). The external reference voltage (figure, lower
right) has drifted by 400 ppm after a total dose
of 200 Gy TID with no consequences for the
operation of the ADC
ADC ADS774KU, OPamp OPA4234UA, Memory
TC554001-70 Current consumption.
Several levels of protection against Single Event
Latch-up ( SEL ). The analog circuits WorldFIP
chipset are protected by resettable fuses
PolyfusesSMD0805P020TS and SMD1210P010TS from
Wickmann . The ADC logical circuit supplies are
individually protected by 80mA linear LDO ( Low
Drop Out ) Micrel voltage regulator
(MIC5203-5.0BM4) with Current Thermal
limiting. A remote shutdown of Micrel regulators
from WorldFIP control is as well
possible. Finally, if the Gateway communication
is interrupted, a Watch-Dog circuit restarts the
FIP interface, putting the chip supply to the
ground level.
The LM35DZ precision thermometer has not shown
any degradation during the run up to 200Gy and
the standard deviation at 10mV/C was identical
after a calibration control measurement.
RADFET Current as a function of dose
 
uFIP Philips VY27257 Fieldrive NS- CG02733AC2
current consumption
AD680JR ADC Ref voltage stability
During irradiation up to 200 Gy (Co-60, 50
Gy/hr), the current has changed by 60 nA.
WorldFIP chipset current consumption suddenly
increases, TID is closed to 200Gy. At this time
the WorldFIP communication has no trouble.
During the test, the temperature has changed by 7
degrees and a thermal cycle after irradiation
test confirmed that the thermo compensation of
the source remained indeed fully operational.
S2
The 12 bit data is transmitted to a WorldFIP
fieldbus bus interface which continues to operate
correctly up to 200 Gy in a standalone operation
mode with a bus access frequency of 100Hz. Two
bilateral CMOS switches (TC4S66F, Toshiba) are
used to select the sensors and to short-circuit
the PIN diodes and RADFETs. The TC4S66F is
limited to a TID of 200 Gy when powered at 11V.
Temp variation as a function of dose
Conclusions
Electric Characteristics of RADFETs

Experimental Results with remote readout
RadTol design, 200Gy in a mixed field. The thermo
compensated current source LM234-Z3 gives good
performances during irradiation. For Single Event
Latch-up protection, resettable fuses could be an
alternative to Micrel regulators for currents
gt50mA Powered at 11V, the Roff resistivity of
TC4S66F switches starts to decrease from 200Gy.
The ADC ADS774KU precision starts to degrade
from 160Gy TID, Ref AD680JR remains in 400ppm
stability. The WorldFIP chipset current supply
shows a 30 increase from 160Gy, TID should be
close to 200Gy.
An important aspect is the readout protocol that
is used to readout the RADFETs. The readout
protocol should not be changed during irradiation
and one must assure that the current in the
device has stabilized at the moment the threshold
voltage is read. The readout current also has an
influence on the temperature coefficient. In the
table presented here, a large number of RADFETs
have been readout at 10mS after the power
switching.
1000nm NMRC RADFET Temperature compensated (non
irrad device)
2.44mV 1bit ADC
NMRC RADFETs comparative calibration curves
produced with the Silicon Dosimeter in a pure
gamma environment from a CO-60 source.
The sensors temperature variation during the run.
100nm NMRC RADFET calibration curve, two 50 Gy
runs.
Tcoef.(Temperature coefficient) The Tcoef of a
RADFET varies as a function of accumulated dose,
current variations and depends on the type of
RADFET. For the 400 nm (300/50 W/L) and the 100
nm (300/50 W/L) RADFETs from NMRC for example,
the T coef. at 8.7mA constant current, changed
to -2.5 mV/ºC and -0.9 mV/ºC after 200 Gy (Co-60
irrad). A LM35DZ precision thermometer
associated with an OPA4234UA OpAmp adjust the
thermal variation over 20C in a range smaller
than 1 bit ADC resolution. The PIN diode at 1mA,
has a T coef. of -2.4 mV/ºC (not irradiated) and
-3 mV/ºC (after 5x1012 1 MeV n/cm2), so that it
can be thermo compensated with the same method.
The current rise time of a TN502P RADFET increase
with dose.
BPW34FS calibration run with 1 MeV neutrons
High flux operation Calibration 9.1x1010
neutrons per mV No data available below 1x1010
n/cm2 A small annealing at room temperature.
30mS
2mS
200 Gy
0 Gy